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Hello. Exhaust systems can make a variety of sounds that we would not intuitively expect. When I was first learning about exhaust systems I was surprised to find out how dynamic they can be. I am assuming you are speaking of the exhaust manifold since the intake manifold doesn’t usually make a noise that sounds like a knocking, but the exhaust frequently can.

When your car is not in drive, the exhaust pressure is much less than when it is in gear. Putting your car in gear adds a load to the motor that can be felt in the exhaust system. Once you begin to move, the flow increases and the exhaust can suck in air instead of pushing it out. This is known as the venturi effect, just like how a carburetor pulls fuel into a motor. Of course, newer cars don’t use carburetors anymore. When you hear the noise, the exhaust is pushing out air. When the sound goes away, exhaust air flow increases enough to create the venturi effect.

Another phenomenon that occurs in exhaust systems is they can leak when they are cold, expand and seal themselves once they warm up. This could be another part of what you are experiencing. As for where the sound is coming from, sounds can be tricky to locate. A big part of car design and engineering is managing all the resonances that transfer through your car. Engine mounts, shocks and struts, brakes and many more components are all part of systems in place to manage noises and vibration. If you hear the sound from inside the car, it is very difficult to be sure where exactly the source of the sound is. I have spent quite a bit of time chasing sounds on one end of a car only to figure out the source is at the other.

If you would like a second opinion, a professional from YourMechanic can inspect the noise for you so that you can have this corrected properly.

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